With existing forms of propeller control systems a compromise of the gains for speed, phase and beta control is made. Specifically, the gains are selected to produce a satisfactory stability margin for all operating conditions and in particular all airspeeds. This in fact results in lower gains being utilized than the gains which would produce optimum control.
Propeller synchrophasing systems serve to control the phase difference in the rotary motion of two propellers so as to maintain it at a desired phase difference. A phase sensor senses the phase difference between the two propellers and this is compared with a desired phase difference to produce a phase error signal. This error signal is then used to vary the relative speed of rotation of the propellers so as to adjust the phase difference between them until it matches the desired phase difference, at which point the error signal is zero. The rate of adjustment of the phase difference is determined by the gain of the system.
U.S. Pat. No. 5,093,791 discloses a propeller synchrophasing system of this known type in which the gain of the system is varied in accordance with the phase difference between the propellers so that there is a lower gain within a limited range of phase difference either side of zero error signal, and there is a higher gain outside of this range. The system is therefore more responsive to higher values of phase difference.